Formation fluids including oil and gas produced at a wellhead are conveyed through flow lines to remote gathering stations. It is conventional practice to use surface safety valves which are responsive to a number of different changes in operating conditions to automatically shut off flow at the onset of unusual or unscheduled operating conditions. For example, catastrophic failures may occur in which flow lines are broken by explosion, storm damage and the like, creating sudden decreases in flow line pressure. Such safety valve installations are designed to automatically close in response to flow line fluctuations either above or below predetermined settings, such as high and low liquid levels, high and low temperatures, and electrical power loss. Conventional safety valves adapted for surface flow line installation are described and illustrated in the Composite Catalog of Oilfield Equipment and Services, published by World Oil, Houston, Tex.